The present invention relates to the preparation of acetaldehyde from hydrogen, carbon monoxide, and methanol under carbonylation conditions. The reaction is conducted in the presence of a catalyst system comprising cobalt, platinum, and iodide.
It is known that cobalt catalyzes the formation of acetaldehyde from methanol, carbon monoxide, and hydrogen. For example, it was disclosed by Wender et al., Science, 113, 206-7 (1951), that a cobalt carbonyl catalyst system could be used. However, the product of the disclosed process was primarily ethanol, together with a small amount of acetaldehyde. It was later shown that the addition of iodide to a cobalt-containing catalyst system increased the amount of acetaldehyde which was produced, although ethanol was still the major product (see, for example, T. Mizoroki et al., Bull. Chem. Soc. Japan, 37, 2, 236-41 (1964)). U.S. Pat. No. 4,239,925 also discloses a cobalt-iodide catalyst system in which small quantities of acetaldehyde were produced along with the desired ethanol product. It has additionally been disclosed that trivalent phosphorous, nitrogen, arsenic, or antimony compounds may be used to further increase the production of acetaldehyde in the presence of cobalt-iodide catalyst systems (see, for example, U.S. Pat. Nos. 4,151,208 and 4,239,704).
Various transition metals, especially the metals of Group VIII, have been employed in cobalt-iodide catalyst systems. However, the objective and the result has been to achieve improvements in the production of ethanol. For example, U.S. Pat. Nos. 4,133,966 and 4,239,924 disclose the addition of ruthenium to a cobalt-iodide catalyst system. It has been reported that the addition of sodium iodide to a cobalt-ruthenium-methyl iodide catalyst system retards the hydrogenation of acetaldehyde to ethanol (see T. Mizoroki et al., Bull. Chem. Soc. Japan, 52, 2, 479-82 (1979). However, the primary product continues to be ethanol. U.S. Pat. No. 4,111,837 discloses that the addition of rhenium to a cobalt-iodide catalyst system also improves ethanol production. The rate of formation of ethanol was also improved by the addition of rhodium and boron to a cobalt-iodide catalyst system (U.S. Pat. No. 4,171,461).
Each of the prior art references mentioned above reports acetaldehyde production at low to moderate chemical yields and production rates. Large amounts of by-products, such as acetic acid, ethyl acetate, methyl acetate, etc., were produced. Also, in each of the references, ethanol was the primary product.
U.S. Pat. No. 3,856,856 discloses that carboxylic acids and esters thereof can be produced by contacting alcohols with carbon monoxide at elevated temperature and pressure in the presence of a catalyst system comprising cobalt, iodide ion, and a platinum promoter. The product mixture obtained by the described process comprises almost totally acetic acid and methyl acetate, with acetaldehyde being present, if at all, in only minute quantities.
Thus, there existed a need for a catalyst system which would provide improved selectivity for acetaldehyde at increased production rates. It has now been found that a cobalt-platinum-iodide catalyst system provides the desired selectivity for acetaldehyde in the reaction of methanol with carbon monoxide and hydrogen (synthesis gas).